Commercial telephone communications, such as public safety systems, are a wireless version of trunking. This sharing of communication paths is managed automatically by a computer and channel selections, and other decisions normally made by the radio user are made by the central controller in the computer. Typically, channel assignment is automatic and completely transparent to an individual user. Trunking offers many benefits, including faster system access, better channel efficiency, and more user privacy and the flexibility to expand. Because of its flexibility, a trunked system can expand to accommodate a growing number of users and restructuring of talk groups. By using trunking the users need not share a common radio frequency channel and compete for air time.
In VHF and UHF frequency ranges trunking efficiency does not exist since there is one licensed frequency and one set of users for a given communication network. In this type of systems users cannot share the frequency channel, as users can in a trunking system. However, the FCC (Federal Communication Commission) has now permitted that a single channel (which is 25 kHz) can be broken into four 6.25 kHz channels. This now allows a user with a 25 kHz channel license to break it into four channels by using trunking. A desirable feature in trunking systems is talk around, which does not require a user to communicate via the trunking system. Talk around is the ability of two subscriber units to directly talk to one another without going through the communication network. Talk around is necessary when either one or both of the subscriber units is outside a coverage area of the communication network. It is to be understood that the coverage area of the communication network is that area in which the subscriber units can communicate with a base unit. Areas in which the subscriber unit cannot communicate with the base unit would occur at a predetermined distance away from the base unit and its transmitter tower, and within areas where the signal is blocked even though these areas may be close to the base unit. For example, this can occur inside of buildings and other structures.
In newer 800 MHz communication systems, a number of channels have been set aside for the use of talk around. But these channels are shared by all users of the frequency band making channel availability a problem. Also, in the older UHF and VHF communication systems, there is no provision for separate channels to handle talk around.
Thus, there exists a need in the art for a system which can provide talk around without using a separate dedicated channel. It is a drawback to the prior systems that when talk around is allowed on the regular frequency channels that are being used by other subscriber units, the talk around subscriber units cause interference, and can actually bring down the entire communication network.